1. Field of Invention
This invention relates to a sampling type measuring device which measures an analog input signal by sampling an analog input signal at a certain period, converting each sample value into digital form, and then processing the digital values.
2. Description of the prior art
The prior art will first be described, taking as an example, a sampling type wattmeter. In conventional wattmeters, an effective power value is obtained by sampling an input voltage waveform and an input current waveform by using an analog-digital converters (hereinafter referred to as AD converter) for converting an analog signal to digital form and multiplying together the sampled values. Therefore, the obtained power value involves an error if the integral multiple of one cycle of the input waveform is out of accord with the integral multiple of a sampling period.
The reason for the error will be discussed with reference to FIG. 7. In conventional sampling wattmeters, the effective power value W (the mean value of instantaneous power values, Vn.multidot.In) is obtained by the following arithmetic averaging calculations. ##EQU1## wherein Vn is the sampled value of input voltage, In is the sampled value of input current, and N is the sampling number, that is the number of samples taken.
In the waveform shown in FIG. 7, one cycle Tin of the input waveform is out of accord with the integral multiple of a sampling period T.sub.AD, that is EQU n1.multidot.Tin.noteq.n2.multidot.T.sub.AD
wherein n1=1,2, 3 . . . ; and n2=3,4, 5 . . . . However, there exists the relationship n2.gtoreq. 2n1+1. This condition prevents sampling of the input waveform at identical phase positions. Thus, a fraction TY appears , as shown in FIG. 7. This fraction TY causes a corresponding error in the effective power value W obtained by arithmetic averaging calculation of Equation (1).
Similar to the case of power measurement, in the measurement of the mean value of rectified waveform, or effective value of an analog input signal by sampling, if a fraction such as TY shown in FIG. 7 appears, there arises a corresponding error.
To reduce adverse influence due to the fraction TY, the following techniques may be used.
1. To regulate the sampling period T.sub.AD relative to the input waveform such that n1.multidot.Tin=n2.multidot.T.sub.AD.
2. To sample the input signal or waveform over several cycles until the following relationship is reached: n1.multidot.Tin=n2.multidot.T.sub.AD.
3. To shorten the sampling period T.sub.AD to considerably decrease the fraction TY relative to one cycle Tin of the input waveform, thereby decreasing the error.
4. To increase the sampling number N (see equation (1)) to relatively decrease the influence due to fraction TY.
The foregoing four methods, however, have the following disadvantages.
1. In the first method, a phase-locked circuit for regulating the sampling period T.sub.AD must have a wide frequency variable range. Thus, the circuit configuration becomes complicated, and leads to high cost.
2. In the second method, measurement cannot be done before an interval corresponding to the integral multiple of one cycle Tin of the input waveform comes into accord with that corresponding to the integral multiple of the sampling period T.sub.AD. Thus, the response time of measurement becomes long.
3. In the third method, an AD converter must be of the high speed type in order to successively process the values picked up at short intervals. Thus, the circuit costs increase.
4. In the fourth method, where the input waveform changes from one value W1 to another W2, the second value W2 cannot be measured quickly in time.
The reason for the above defect will be described by taking the condition that an effective power value W1 has been obtained by processing, in accordance with equation (1) with 1000 values picked up by 1000 samples. In this case, if the input waveform changes to a different value W2 after the sampling number reaches 1000, the then calculated value cannot easily reach the new value W2 because the influence of the sum of the values corresponding to the first to the thousandth samples is significant. That is, since the sum of the values corresponding to the first to thousandth samples is very large, even when the then sampled values corresponding to the thousandth and first and subsequent samples are added to the last sum in equation (1), the sum (.SIGMA.) changes little. Thus, a large number of samples must be obtained until the new value W2 is actually calculated.